This application claims priority to an application entitled xe2x80x9cApparatus and Method for Providing Closed-Loop Transmit Antenna Diversity in Mobile Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Oct. 9, 1999 and assigned Serial No. 99-43679, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to an apparatus and method for providing transmission antenna diversity on a downlink, and in particular, to an apparatus and method for providing closed-loop transmission antenna diversity by adaptively applying weights to closed-loop transmission diversity according to the channel environment in a mobile communication system.
2. Description of the Related Art
CDMA (Code Division Multiple Access) systems have evolved from the conventional voice transmission-based mobile communication systems to the new IMT-2000 standard, which provides such additional services as transmission of high quality voice, moving pictures, and Internet browsing.
For provision of these various services, the capacity of the downlink must have a higher gain for the increased traffic. For a mobile terminal that is moving at a low speed, it is known that a base station using antenna diversity can have gain about 1 to 7 dB higher than a base station which is not using antenna diversity in a CDMA mobile communication system. This implies that system capacity can be increased by two or three times and that a receiver of the terminal has a high enough gain.
The transmit antenna diversity is a scheme of transmitting a signal to a terminal through at least two transmission antennas from a base station. Two approaches may be taken when using transmission antenna diversity.
One of the approaches is a closed-loop scheme (feedback type transmission diversity). The mobile terminal estimates the strengths of signals received from the base station antennas and transmits weight information for each antenna to the base station. After changing weights for transmission power and phase based on the weight information, the base station transmits data.
The other approach is an open-loop scheme. The base station allocates equal transmission power to each antenna and transmits data through the antennas with different orthogonal codes.
The following description concerns the closed-loop transmit antenna diversity scheme.
A base station as described hereinbelow belongs to an UMTS (Universal Mobile Telecommunication System). While the base station may have more than two transmission antennas, two transmission antennas are assumed in the following description for greater clarity.
If the base station transmits signals through a plurality of antennas, i.e., with transmit antenna diversity, the reception BER (Bit Error Rate) is decreased at the terminal. For downlink transmission through the antennas, the base station allocates a unique weight to each antenna transmission signal. The antenna-specific weight should be set in such a way that the terminal can receive the next antenna signal with maximal power. If the terminal estimates the downlink channel environment and notifies the base station of the estimation, the base station can allocate an optimum weight to each antenna. To this end, the mobile terminal estimates the status of a channel on which a signal is received from the base station and transmits the estimated channel status (i.e., channel environment) to the base station.
FIG. 1 is a block diagram of a transmitting device with transmit antenna diversity in a general mobile communication system.
Referring to FIG. 1, reference numerals 101 and 111 denote a primary common pilot channel (P_CPICH) and a secondary common pilot channel (S_CPICH), respectively. The base station uses one P_CPICH 101. It can also generate a plurality of S_CPICHs 111 to transmit on the downlink with feedback-mode transmit diversity. The P_CPICH 101 and S_CPICHs 111 are all 1 s and spread with one of orthogonal variable spreading factor (OVSF) code with Spreading Factor (SF) 256. The P_CPICH 101 is scrambled with a primary scrambling code and the S_CPICH 111 is scrambled with a secondary scrambling code. Both the common pilot channels 101 and 111 are transmitted at 300 bits per 10-ms frame.
A spreader 103 spreads the P_CPICH 101 and a spreader 113 spreads the S_CPICH 111. A multiplier 104 scrambles the spread P_CPICH 101 with a primary scrambling code. One primary scrambling code is assigned to each base station, in order to identify the base station. A multiplier 114 scrambles the spread S_CPICH 111 with the secondary scrambling code. An encoder 133 subjects a downlink dedicated physical data channel (DPDCH) 131 to encoding and rate matching. An interleaver 135 interleaves the downlink DPDCH received from the encoder 133. A multiplexer (MUX) 137 multiplexes a TPC (transmit power control) 136, a TFCI (Transmit Format Combination Indicator) 138, and the interleaved DPDCH. The TPC 136 is used to control signal transmission power and the TFCI 138 provides information about the channel encoding method and transmission rate of the data. A MUX 141 multiplexes the multiplexed DPDCH, TFCI, and TPC received from the MUX 137 and a pilot signal 132 for antenna #1 as indicated by reference numeral 180. A MUX 151 multiplexes the multiplexed DPDCH, TFCI, and TPC received from the MUX 137 and a diversity pilot signal 134 for antenna #2 as indicated by reference numeral 181. The pilot signals 131 and 134 typically have the same pilot pattern but may have different pilot patterns.
A spreader 143 spreads the output of the MUX 141 with an assigned OVSF code. A multiplier 144 scrambles the output of the spreader 143 with the primary scrambling code or the secondary scrambling code. The secondary scrambling code is used instead of the primary scrambling code when there is lack of OVSF codes assigned to channels using the primary scrambling code. A multiplier 145 multiplies the output of the multiplier 144 with a weight 175 assigned to antenna #1180. A summation device 160 sums the P_CPICH of antenna #1180 received from the multiplier 104 and the DPDCH received from he multiplier 145. The sum is filtered by a filter 162, modulated to a radio signal y an RF module 164, and transmitted to the terminal through antenna #1180.
A spreader 153 spreads the output of the MUX 151. A multiplier 154 scrambles the output of the spreader 153 with the primary scrambling code or the secondary scrambling code. A multiplier 155 multiplies the output of the multiplier 154 with a weight 174 assigned to antenna #2181. A summation device 161 sums the S_CPICH received from the multiplier 114 and the DPDCH received from the multiplier 155. The sum is filtered by a filter 183, modulated to a radio signal by an RF module 185, and transmitted to the terminal through antenna #2181.
A weight generator 171 generates weights 174 and 175 for antennas #1 and #2 based on downlink channel status information received from the mobile terminal and feeds them to the multipliers 155 and 145, respectively. The weights 174 and 175 are expressed in complex vectors and one of them has a fixed value.
FIG. 2 is the format of a feedback signal message by which the terminal transmits the downlink channel status information to the base station.
Referring to FIG. 2, the feedback signal message is transmitted on an uplink dedicated physical control channel (UL_DPCCH), at a rate of ten bits per slot. Specifically, the feedback signal message includes a pilot field 201, a TFCI field 202, a FBI (Feedback Indicator) field 203, and a TPC field 204. These fields occupy a total of ten bits. The length of each field in the feedback signal message varies depending on the channel environment.
The channel environment is indicated by presence or absence of the TFCI field 202 and the FBI field 203. According to the channel environment, the pilot field 201 can be five to eight bits. For example, the TFCI field 202 is used when the base station and the terminal transmit channels with different SFs at the same time. In this case, the TFCI field 202 of an uplink dedicated physical channel is two bits. The TPC field 204 is used to control the power of a downlink channel, occupying one or two bits.
The FBI field 203 is used to transmit feedback information about SSDT (Site Selection Diversity Transmission) and feedback type transmission diversity. The SSDT is information indicating which cell is transmitting the highest power signal to the mobile terminal during soft handoff. The FBI field 203 is one or two bits. In the case of a one-bit FBI, this implies that one of the SSDT and the feedback transmit diversity is used. In the case of a two-bit FBI, this implies that both the SSDT and the feedback transmit diversity are used. The FBI field 203 includes an S field for the SSDT and a D field for the feedback transmit diversity. One bit is assigned to each of the S and D fields. If the SSDT is not used, the D field for the feedback transmit diversity can be two bits (the S field is used for the feedback transmit diversity).
FIG. 3 illustrates a channel environment information set transmitted to the base station in the D field of the FBI field 203 by the terminal. The channel environment information set is four bits. Of this, only one bit of the channel environment information is transmitted per slot. Therefore, the channel environment information set is transmitted over four slots. In order to transmit the channel environment information set to the base station, the terminal estimates the channel environment between the base station and the terminal from the P_CPICH received through antenna #1180 and the S_CPICH received through antenna #2181 of the base station.
The FSM bits shown in FIG. 3 are transmitted bit by bit through the D field for the feedback type transmit diversity in the FBI field of the uplink DPCCH. The length of the channel environment information set is the sum of lengths of the power weight NPO 301 and the phase weight NPH 303. The NPO 301 is one bit and the NPH 303 is three bits. The NPO 301 precedes the NPH 303 in the order of transmission. Referring to FIG. 3, transmission of the channel environment information set starts with its MSB (Most Significant Bit) and ends with its LSB (Least Significant Bit).
Table 1 and Table 2 list the binary channel environment information set transmitted in the D field of the FBI field, which is used for weight generation by the transmitting device of the base station shown in FIG. 1. Table 1 shows the MSB of the channel environment information set. In Table 1, FSMPO=0 indicates that estimated channel power ratio of the two antennas is approximately 0.2:0.8, FSMPO=1 indicates that estimated channel power ratio of the two antennas is approximately 0.8:0.2. Table 2 shows the difference in phase, which is estimated at the mobile station, between a signal transmitted through an antenna selected as a reference antenna and a signal transmitted through another antenna at the base station.
The channel environment information set represents the indexes of values as listed in Table 1 and Table 2, which are the most approximate to a downlink channel environment estimation value obtained from downlink channel signals received in the mobile terminal from antennas #1 and #2 of the base station.
As shown in Table 1 and Table 2, weights to be assigned to the transmission antenna are preliminarily agreed between the terminal and the base station in the conventional technology of feedback-mode closed-loop diversity.
In the following description, let the reference antenna be antenna #1 and the transmission power of a downlink channel signal be normalized to 1.
The terminal receives the P_CPICH and the S_CPICH from antenna #1 and antenna #2 of the base station, respectively, and estimates the phase difference and power of each common pilot channel. If it turns out that the phase difference between the P_CPICH and the S_CPICH is 30xc2x0 and the power levels of the P_CPICH and the S_CPICH are 0.7 and 0.3, respectively, the terminal determines a power index of 1 (FSMPO=1) with 0.8 for the power of the P_CPICH of antenna #1 and 0.2 for the power of the S_CPICH of antenna #2, approximate to the estimation values, referring to Table 1. Then, the terminal determines a phase index 111 of 45xc2x0 as the phase difference approximate to the estimation value of 30xc2x0. The power index and the phase index form a channel environment information set. The mobile station inserts each bit of the channel environment information set, starting from the MSB, into the D field of the FSM (Feedback Signaling Message) field and thus transmits the channel environment information set to the base station over four slots.
FIG. 4 illustrates signals expressed in vectors, where such signals are received in the mobile terminal from a plurality of antennas of the base station, and illustrates weight vectors of the channel environment information set as shown in Table 1 and Table 2, on a coordinate plane. A case is assumed where there are two antennas in the base station and transmit signal power control is implemented for each antenna.
In FIG. 4, a channel environment information set related with phase shown in Table 2, xe2x80x98000xe2x80x99 represents a vector 401, xe2x80x98001xe2x80x99 represents a vector 402, xe2x80x98011xe2x80x99 represents a vector 403, xe2x80x98010xe2x80x99 represents a vector 404, xe2x80x98110xe2x80x99 represents a vector 405, xe2x80x98111xe2x80x99 represents a vector 406, xe2x80x98101xe2x80x99 represents a vector 407, and xe2x80x98100xe2x80x99 represents a vector 408. The terminal transmits to the base station the index of a phase vector most approximate to the phase difference between the signals received through antennas #1 and #2 as the phase-related channel environment information set to the base station. Vectors 421 to 428 are obtained in case the channel environment information set related with the transmit signal power control of each antenna indicates 0.2 for the signal power of each antenna in Table 1. In this case, the signal power of the antennas is controlled to 0.8 and 0.2.
A vector 411 indicates the transmit signal of antenna #1 received at the terminal through its antenna at time t=T. A vector 416 indicates the transmit signal of antenna #2 received at the terminal through its antenna at time t=T. To render the strengths of signals received at the terminal in relation to vectors 411 and 426 maximal, the difference of the phases between the vectors 411 and 426 should be minimal.
To minimize the difference in phase between the transmit signals of antennas #1 and #2 received at the terminal, the base station and the terminal calculate weight vectors to be assigned to the antennas in the following manner. If the base station designates antenna #1 as a reference antenna, the terminal calculates the phase difference between the vector 411 of the transmit signal of antenna #1 and the vector 426 of the transmit signal of antenna #2 based on the vector 411. Since the phase difference between the vectors 411 and 426 is shown to be 40xc2x0 in FIG. 4, the terminal transmits the index xe2x80x98111xe2x80x99 of a vector with the phase difference 45xc2x0 approximate to the actual phase difference 40xc2x0 as a phase-related channel environment information set to the base station.
The channel environment information set is used when the base station sets a weight for each antenna to be used at time t=T+1. The base station maintains a current weight for antenna #1 and assigns a weight decreased in phase by 45xc2x0 from a current weight for antenna #2. In the above conventional method, the weight of the reference antenna is fixed and that of another antenna is varied.
Let a signal to be transmitted from the base station be s[n]. s[n] is a spread signal that becomes L signal sequences after operated with weight vectors W as many as the antennas of the base station. L is the number of the dedicated antennas used in the base station and the number of the weight vectors W assigned to the antennas is (Lxc3x971). Thus an output signal x[n] of a diversity transmission antenna is computed by
x[n]=Ws[n]xe2x80x83xe2x80x83(1)
If the output signal of an ith antenna among the diversity transmission antennas is
xi[n]=Wis[n]xe2x80x83xe2x80x83(2)
The discrete-time multi-path channel output equation of the ith antenna signal can be given by
yi[n]=hi,0[n]xi,[n]+hi,1[n]xi[nxe2x88x921]+ . . . +hi,Mxe2x88x921[n]xi[nxe2x88x92(Mxe2x88x921)](i=1,2, . . . ,L)xe2x80x83xe2x80x83(3)
where yi[n] is a signal received at the terminal from the ith antenna, hi,0, . . . , hi,Mxe2x88x921 are the coefficients of the ith channel, and M represents the number of fingers of the terminal. Therefore, the antenna of the terminal receives a signal expressed as
y[n]=y1[n]+y2[n]+y3[n]+ . . . +yL[n]+n[n]xe2x80x83xe2x80x83(4)
where n[n] is channel noise.
When the auto-correlation function of a spreading sequence with which the signal s[n] is spread is close to an impulse sequence, a despread signal formula is
rm[p]=(h1,mW1+h2,mW2+ . . . +hL,mWL)b[p]+um[p](m=0, 1, 2, . . . , Mxe2x88x921)xe2x80x83xe2x80x83(5)
where rm[p] is the output of an mth correlator in a rake receiver with the input signal y[n], b[p] is a data symbol, um[p] is noise after despreading, WL is the weight vector, and M is the number of correlators (fingers) in the rake receiver. Eq. (5) can be expressed in a matrix as
r[p]=[r1[p]r2[p] . . . rM[p]]Txe2x80x83xe2x80x83(6)
u[p]=[u1[p]u2[p] . . . uM[p]]Txe2x80x83xe2x80x83(7)
From Eq. (6) and Eq. (7), a despread signal at the terminal is defined as
xe2x80x83r[p]=Hwb[p]+u[p]xe2x80x83xe2x80x83(8)
where H is the channel estimation matrix with size Mxc3x97L (M is the number of correlators in the rake receiver of the terminal and L is the number of antennas in the base station).
The coefficient Hw preceding the data symbol b[p] in Eq. (8) influences the SNR (Signal-to-Noise Ratio) of the input signal r[p] at the terminal. Since Hw is a channel estimation matrix and H is a variable depending on channel environment, the base station cannot control H. However, it is possible for the base station to control a weight vector w based on feedback information received from the terminal. Accordingly, the SNR of the input signal r[p] can be increased by optimizing the weight vector w.
To achieve an optimum weight vector,
Wk=arg max wHHkHHkw//w//2=Pkxe2x80x83xe2x80x83(9)
where wH and HkH are the conjugate transpose matrices of w and H, and Pk is the total transmission power of signals transmitted from all the transmission antennas. In calculating a weight by Eq. (9), since the weight vector w is an Lxc3x971 complex matrix, the terminal should transmit 2L real numbers representing the weight vector.
In conclusion, when the differences in power and phase between a P_CPICH and an S_CPICH estimated by a terminal are not shown in Table 1 and Table 2, an error occurs between a weight assigned to each antenna in a base station based on a feedback signaling message received from the terminal and a real weight with which the receive signal power of the terminal is maximized in the conventional closed-loop transmit antenna diversity scheme. As a result, the performance deteriorates.
It is, therefore, an object of the present invention to provide an apparatus and method for adaptively calculating optimum weight vectors for the current transmission stage using optimum weight vectors used at the previous transmission stage in a predetermined period and assigning a weight to each base station antenna based on the obtained weight vectors.
It is another object of the present invention to provide an apparatus and method for receiving signals from a plurality of base station antennas and calculating optimum weight vectors referring to a look-up table listing differential weight vectors in a terminal, so that the sum in vectors of any signals received from the base station antennas is maximal.
It is a further object of the present invention to provide an apparatus and method for transmitting the indexes of differential weight vectors shown in a look-up table to provide adaptively calculated differential weight vectors as channel status information to a base station by a terminal.
It is still another object of the present invention to provide an apparatus and method for initializing adaptively obtained weight vectors at every predetermined interval and then recalculating optimum weight vectors.
The above objects can be achieved by providing an apparatus and method for providing closed-loop transmit antenna diversity in a mobile communication system. To maximize the sum of the vectors of signals received from a base station having at least two antennas through the at least two antennas, a terminal calculates the vectors of the signals received through the antennas of the base station, calculates differential vectors for the input signal vectors to maximize the sum of the input signal vectors, and transmits information about the differential vectors to the base station to maximize the sum of the vectors of signals received at the next time instant.